Localization of gray matter/white matter borders is critical to the accurate placement of electrodes for deep brain stimulation (DBS). These neuroanatomical borders are traditionally identified using CT and MR imaging, aided by microelectrode recording (MER) techniques. This project will evaluate the ability of an intracranial probe utilizing near-infrared (NIR) spectroscopy to identify differences between gray and white matter to localize small targets as required for placement of DBS leads and other stereotactic surgeries. Localization with the NIR probe will be validated with standard MER techniques as well as with perioperative imaging. The time required for use of the NIR probe will be compared to that of MER. If validated, use of this NIR probe would lessen the difficulty of intraoperative localization. There will be three specific aims: (1) the NIR probe will first be used in a rodent model, where pilot data has verified the ability to accurately detect the white matter track of the corpus callosum. Sites with different white matter thickness will be used to test the resolution of the probe, and NIR data will be compared to post-mortem localization of the probe in histologic sections; (2) an NIR probe has been designed for simultaneous microelectrode recording that will be tested in the rodent model to acquire NIR and MER data simultaneously. This probe will then be used in rodents and humans to more precisely define the ability of the NIR probe to provide localization of the fine layers of gray and white matter. In humans, particular attention will be given to localization of layers at the base of the thalamus and pallidum that is required for definitive placement of DBS electrodes; and (3) refine the development of the NIR method. The proposed studies seek to validate and further develop a new method for precise localization of important subcortical structures commonly encountered in stereotactic surgery for Parkinson's disease.